A clutch disk assembly used in a vehicle has a clutch function for engaging with and disengaging from a flywheel, and a damper function for absorbing and damping torsional vibration from the flywheel. Typical vibrations encountered with vehicles include idling noises (clattering), driving noises (acceleration and deceleration rattling, muffled noises), and tip-in and tip-out (low-frequency vibrations). The damper function eliminates these noises and vibrations.
Idling noises are those that sound like a clattering generated from the transmission when the shifter is put into neutral at a stoplight or the like, and the clutch pedal is released. The cause of these noises is that the engine torque is low, and torque fluctuates considerably during engine combustion near the engine idle speed. At such times the input gear and counter gear of the transmission clash.
Tip-in and tip-out (low-frequency vibrations) are large vibrations along the length of the chassis, produced when the accelerator pedal is suddenly pressed or released. If the power transmission system is low in stiffness, torque transmitted to the tires will be transmitted back from the tires, and this reverberation generates excessive torque at the tires, and this results in longitudinal vibration that strongly shakes the chassis back and forth transiently.
With idling noises, close to zero torque is problematic in the torsional characteristics of the clutch disk assembly, and it is therefore better for the torsional stiffness to be low. On the other hand, with tip-in and tip-out longitudinal vibration, the torsional characteristics of the clutch disk assembly must be made as solid as possible.
To solve the above problem, a clutch disk assembly has been proposed in which two-stage characteristics are attained by using two types of spring members. Here, torsional stiffness and hysteresis torque are kept low in the first stage of the torsional characteristics (the region of low torsion angle), which is effective in preventing noises during idling. Since the torsional stiffness and the hysteresis torque are kept high in the second stage of the torsional characteristics (the region of high torsion angle), tip-in and tip-out longitudinal vibration can be sufficiently damped.
Furthermore, there is a known damper mechanism with which minute torsional vibrations are effectively absorbed without operating a large friction mechanism for the second stage when the minute torsional vibrations are inputted, such as those caused by combustion fluctuation in the engine, in the second stage of the torsional characteristics.
In order for no large friction mechanism to be operated for the second stage when the minute torsional vibrations are inputted, such as those caused by combustion fluctuation in the engine, in the second stage of the torsional characteristics, it is necessary to ensure a gap of a specific angle in the rotational direction between a spring member with high torsional stiffness and the large friction mechanism in a state in which the spring member with high torsional stiffness has been compressed.
The angle of this gap in the rotational direction is very small (only about 0.2° to 1.0°, for example), and it is present at both the second stage on the positive side, where an input plate (input rotary member) is twisted to the drive side in the rotational direction (positive side) with respect to a spline hub (output rotary member), and the second stage on the negative side, where the twist is to the opposite side (negative side).
In particular, since the structure constituting the gap in the rotational direction was achieved in the past by the same mechanism at both the positive second stage and the negative second stage, this rotational direction gap was always generated on both the positive and negative sides of the torsional characteristics, and furthermore the magnitude of the angle was the same.
However, there are cases when it is preferable for the size of the rotational direction gap to be different on the positive and negative sides of the torsional characteristics, according to the characteristics of the vehicle, and it is also conceivable that it might even be preferable not to provide the above-mentioned rotational direction gap on one side of the torsional characteristics.
More specifically, on the negative side of the torsional characteristics, the above-mentioned rotational direction gap is necessary in order to lower the peak of vibration at the resonant rotational speed during deceleration. However, with a front-wheel drive vehicle, a resonance peak often remains in the practical rotational speed band, and if the above-mentioned rotational direction gap is ensured on the positive side of the torsional characteristics, this will adversely affect the noise and vibration performance near the resonant rotational speed.
In view of this, a damper mechanism has been proposed that has a gap for generating low hysteresis torque with respect to minute torsional vibration on only the negative side of the torsional characteristics (see Japanese Laid-Open Patent Application 2002-266943, for example).
However, to obtain a structure having a low hysteresis torque generating gap on just one side of the torsional characteristics, many extremely small friction washers or cone springs are necessary. Accordingly, the number of parts increases, and assembling the parts entails more labor. Specifically with a conventional damper mechanism, employing the above-mentioned structure drives up the manufacturing cost.